This disclosure relates to ranging systems and methods and, in particular, ranging systems that disambiguate distances measured by time-of-flight (TOF) ranging techniques.
A typical TOF system emits a light signal whose intensity oscillates at a single frequency. The system repeatedly integrates the returning “echo” of this signal over a preset fraction (typically ½) of the signal's modulation period, starting and ending at specific modulation phases. At certain times, the system saves a result of the integration, shifts the start and end of the integration time by a preset fraction of the signal modulation period (typically ¼), and resumes the integration. After several repetitions of this procedure, it is possible to calculate from the saved results of the integration the phase shift of the echo signal modulation relative to the original signal modulation. This phase shift is a periodic function of the distance traveled by the light signal, which can be used to measure light travel distances and, by extension, distances to objects reflecting light toward the TOF system. However, the periodicity or “wrapping around” of the phase shift means that multiple different distances of light travel can yield the same phase shift. In other words, the phase-shift-to-distance relation is ambiguous. The ambiguity is removed when it can be said with certainty that only the shortest distance corresponding to each phase shift is plausible, due to, for example, the TOF system being used in an enclosed space where it has no chance to receive an “echo” from objects located further than its so called ambiguity distance, corresponding to 360-degrees phase shift. If no other means to avoid or remove the distance ambiguity are employed, the ambiguity distance must be considered the maximum range of the TOF system. The ambiguity distance is inversely proportional to the light intensity modulation frequency, and therefore it can be lengthened by lowering the frequency. However, doing so may worsen the accuracy of distance measurements done by a TOF system.